Time of flight mass spectrometer and multiple detector therefor

ABSTRACT

An ion detection arrangement  140  for a time-of-flight (TOF) mass spectrometer  10  includes a beam splitter formed as a mesh  150  at the end of the TOF acceleration and detection chamber  110 . Ions enter the detection arrangement through a common entrance window and are then divided by the beam splitter. Those ions striking the mesh  150  generate secondary electrons  160  which are detected by a microchannel plate forming a first detector  170 . Those ions passing through the ion beam splitter are detected directly by a second detector  190  also formed from a microchannel plate. 
     The two detectors are each connected to a corresponding data acquisition system  180, 200  and the data obtained by each are combined to generate a mass spectrum. The problems of detector saturation are thus avoided.

FIELD OF THE INVENTION

The invention relates to a time of flight mass spectrometer (TOFMS) andin particular to a detector arrangement having a plurality of detectorsfor TOFMS.

BACKGROUND OF THE INVENTION

Time of flight mass spectrometry (TOFMS) allows the rapid generation ofwide range mass spectra. TOFMS is based upon the principle that ions ofdifferent mass to charge ratios travel at different velocities such thata bunch of ions accelerated to a specific kinetic energy separates outover a defined distance according to the mass to charge ratio. Bydetecting the time of arrival of ions at the end of the defineddistance, a mass spectrum can be built up.

Most TOFMS operate in so-called cyclic mode, in which successive bunchesof ions are accelerated to a kinetic energy, separated in flightaccording to their mass to charge ratios, and then detected. Thecomplete time spectrum in each cycle is detected and the results addedto a histogram.

One of the primary challenges in TOFMS is to maximize the dynamic rangeof the device. This is primarily constrained by the processing of thesignal from the ion detectors: not only must the number of ions arrivingbe counted, but also the time at which the ions arrive. This data mustbe obtained and output before the next set of data can be processed.

The earliest TOFMS devices employed analog to digital converters (ADC)to digitize the output of a DC amplifier connected to a collectorelectrode. The collector electrode in turn received electrons generatedby one or more microchannel plate electron multipliers when ionsimpinged thereon. The output of the ADC was coupled to a charge recorderor oscilloscope and, subsequently, a transient recorder.

Although ADC data acquisition systems do not suffer from the drawbacksof time to digital converters (TDC) (see below), their dynamic range islimited by the non-linearity of the electron multiplier and also by thespeed of the ADC itself. Even a fast ADC (<5 ns sampling rate), forminga first part of a transient recorder, has a limited dynamic range, andbecomes complex, expensive and problematic at the highest massaccuracies demanded. Also, signal variations on the ADC reduce the massaccuracy of the mass spectrometer.

Time to digital converters (TDC) employ ion counting techniques to allowa mass spectrum to be generated. Here, the impact of a single ion isconverted to a first binary value e.g. 1 and the lack of impact isrepresented as a second binary value (e.g. 0). These data can then beprocessed via various timers and/or counters.

The advantage of a TDC over the analogue detection technique describedabove is that the signal output from the electron multiplier in respectof each ion impact is treated identically so that variations in theelectron multiplier output are eliminated. There is, however, a limit tothe dynamic range of a TDC detector, caused by a so-called dead timeassociated with ion detection. The dead time occurs immediatelyfollowing the impact of an individual ion. If a subsequent ion arrivesduring this dead time, it is not recorded. Thus, at higher iondensities, the total of ions arriving may be significantly more than thenumber actually detected.

Several techniques have been proposed in recent years to address theproblems inherent with ADC and TDC ion detection techniques.WO-A-98/40907 discloses an integrated TDC/ADC data acquisition systemfor TOFMS. A logarithmic (analogue) amplifier is arranged in parallelwith a TDC and also an integrating transient recorder. The TDC cancollect data and analyse it in respect of very small ion concentrationswhilst the transient recorder is able to collect and analyse data inrespect of much higher ion concentrations without saturation. Thedynamic range of the data acquisition system overall is thus much largerthan that of a traditional TDC without sacrificing sensitivity at lowerion concentrations. However, the problems characteristic of ADCdetectors identified above still remain at higher ion concentrations.

Another arrangement is disclosed in an article by Kristo and Enke, inRev. Sci. Instrum. (1988) vol. 59/3, pages 438-442. The arrangementcomprises two channel type electron multipliers in series, together withan intermediate anode. The intermediate anode intercepts the majority ofelectrons generated by the first multiplier and allows these minority ofelectrons which are not intercepted to be captured by the secondelectron multiplier. An analog amplifier generates a first detectoroutput from the anode, and a discriminator and pulse counter generates asecond detector output from the second electron multiplier. The outputsof the two detectors are then combined. This technique also suffers fromthe problems associated with a combined TDC/ADC system.

An alternative approach to the issues of sensitivity and dynamic rangeis set out in WO-A-98/21742. Here, an array of adjacent but separateequal area anodes is employed, with a separate TDC for each anode. Thisallows parallel processing of incoming ions, to increase the number ofsimultaneously arriving ions that are detected and thus to increase thedynamic range. The problem with this, of course, is that increases inthe number of detectors increases the cost and, on average, an array ofN detectors can only increase the total number of ions detected by amaximum of N times.

To address this, WO-A-99/67801 discloses the use of two anodes ofunequal area. This extend the dynamic range of the detector since, withlarge numbers of a particular ion specie arriving at the detector, theaverage number of ions detected on the smaller anode is small enough toreduce the effects of saturation. The larger anode, by contrast, candetect ions arriving with a lower concentration without an unacceptableloss of accuracy.

WO-A-99/38190 and WO-A-99/38191 also each disclose a microchannel plateelectron multiplier having collection electrodes (anodes) with differentsurface areas.

Such multiple detector techniques suffer from drawbacks, nevertheless.Firstly, physical cross-talk between the channels is inevitable. Due tothe spatial spread of electron clouds created by the electronmultipliers, only a part of the cloud may be collected on the smalleranode; similarly partial carry-over of electron clouds from the largercollector can take place. In addition, the close proximity of the anodescauses capacitive coupling between each which in turn increases thelikelihood of electronic cross-talk. The multiplier voltage may collapsewhen very intense ion pulses are received, as is possible in, forexample, ICP/MS and GC/MS. This results in reduced sensitivity forsubsequent mass peaks. Finally, the ratio of “effective areas” maydepend heavily on parameters of the incoming ion beam (which in turn maydepend upon space charge, ion source conditions etc.) which leads to amass dependence upon the ratio. This problem is particularly pronouncedin narrow ion beams such as are produced in orthogonal accelerationTOFMS.

U.S. Pat. No. 5,777,326 addresses the last problem outlined above byemploying a multitude of similar collectors after a common multiplier.Each collector is connected to a separate TDC channel. Whilst thesolution provided by U.S. Pat. No. 5,777,326 does largely remove themass dependence upon the ratio of anode areas, it fails to address theother problems with this multiple detector arrangement and also extendsdynamic range only by a factor equal to the number of channels. Thus,the construction can become complex and even then may not be adequatefor certain applications such as gas chromatography/mass spectrometry(GC/MS).

It is an object of the present invention to address the problems of theprior art.

According to a first aspect of the present invention, there is providedan ion detection arrangement for a time-of-flight mass spectrometercomprising: an ion beam splitter arranged to intercept a first part ofan incident bunch of ions which has passed through the time-of-flightmass spectrometer, but to allow passage of a second part of thatincident bunch of ions; a first detector means arranged to detect ionsincident upon the ion beam splitter; and a second detector meansarranged to detect those ions which pass through the said ion beamsplitter.

The detector of the invention accordingly provides a multiple detectorwherein ions that have passed through a TOFMS enter into the detectorarrangement through a common entrance window and are then divided by anion beam splitter such as a conversion dynode or grid. Those ionsstriking the ion beam splitter generate, in the preferred embodiment,secondary electrons which are detected by a first detector means,whereas those ions passing through the ion beam splitter are detected bya second detector means. The ions are accordingly divided at an earlystage in their detection, and the multiple detector arrangementaccordingly provides greatly reduced electronic and physical cross-talkbetween the detectors. The dynamic range is extended without sacrificeof linearity, and better quantitation is available.

Preferably, the ion beam is divided by the ion beam splitter in anunequal proportion such that the vast majority of ions entering themultiple detector arrangement are either intercepted by the ion beamsplitter, or, alternatively, the vast majority of ions are notintercepted by the ion beam splitter.

It is preferable that the ion beam is divided into two unequal parts sothat one of the detectors continues to operate even when the other issaturated. In preferred embodiments, greater than 90% of the ion beam isallowed to pass through the ion beam splitter which may be, for example,a grid or mesh. Alternatively, less than 10% of the ion beam may passthrough the ion beam splitter so that more than 90% is intercepted byit. The latter arrangement is particularly preferred because it iseasier to manufacture than a largely transparent grid. Also, the latterarrangement allows secondary electrons which may be generated when theion beam strikes the beam splitter to be focussed in time of flight asthey pass towards the first detector means. Electrons are typicallyeasier to focus than incoming ions because electrons are relatively muchlighter and faster than ions so that TOF spreading is correspondinglysmaller.

It is preferable that the ion beam splitter is arranged to split theincoming ion beam in such a way that each detector detects ions frommultiple points uniformly spread over the width of the incoming ionbeam. It is desirable that a representative sample of ions is extractedfrom across the beam width, not just from one particular point.

According to a second aspect of the present invention, there is provideda method of detecting the time of flight of ions in an ion beam of atime-of-flight mass spectrometer, comprising: directing ions to bedetected through the time-of-flight mass spectrometer and toward an ionbeam splitter; intercepting a first portion of the ions in the ion beamat the ion beam splitter; allowing passage of a second portion of theions in the ion beam through the ion beam splitter; detecting ionsintercepted by the ion beam splitter with a first detector means; anddetecting ions passing through the ion beam splitter with a seconddetector means.

Further advantageous features are set out in the dependent claims whichare appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be put into practice in a number of ways, and someembodiments will now be described by way of example only and withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a time-of-flight mass spectrometerincluding a multiple detector representing a first embodiment of thepresent invention;

FIG. 2 shows, in more detail, the multiple detector shown in the time offlight mass spectrometer of FIG. 1;

FIG. 3 shows a second embodiment of a multiple detector for a time offlight mass spectrometer;

FIG. 4 shows a third embodiment of a multiple detector for atime-of-flight mass spectrometer;

FIG. 5 shows a fourth embodiment of a multiple detector for atime-of-flight mass spectrometer, which is a variation of the thirdembodiment of FIG. 4; and

FIG. 6 shows a fifth embodiment of a multiple detector for atime-of-flight mass spectrometer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows, in schematic terms, a time-of-flight mass spectrometer(TOFMS) 10. The TOFMS comprises an ion source shown as a representativeblock 20 in FIG. 1. The ion source may be any suitable continuous orpulsed source, such as an electrospray source, an electron impact sourceor the like. Indeed, the ion source 20 may in fact be an upstream stagein an ms/ms analysis, e.g. a quadrupole mass spectrometer or an iontrap.

Gaseous particles from the ion source 20 enter an extraction chamber 30which is evacuated to a first pressure below atmospheric pressure by avacuum pump (not shown). The ions exit the extraction chamber 30 into anintermediate chamber 40 which is likewise evacuated, but to a lowerpressure than the pressure within the extraction chamber 30, by a secondvacuum pump, again not shown. The ions then leave the intermediatechamber 40 and enter a focussing chamber 50 through a conical inletaperture 60. The focussing chamber 50 contains a series of rods 70 whichreduce interferences from unwanted species and focus the ions so as toreduce the energy spread thereof. Although a quadrupole rod arrangementis shown in FIG. 1, it will be appreciated that hexapole arrangementscan likewise be employed for this purpose.

The rods 70 cause an ion beam 80 to be formed in the focussing chamber50 and this passes towards an orifice 90 in a wall 100 at the end of thefocussing chamber axially distal from the inlet aperture 60 thereof. Aswith the extraction and intermediate chambers 30, 40, the focussingchamber 50 is evacuated to a third pressure still lower than thepressure within the intermediate chamber 40 by a further vacuum pump(again, not shown).

The ion beam 80 passes through the orifice 90 in the wall 100 and intoan acceleration and detection chamber 110. The acceleration anddetection chamber 110 which is shown in FIG. 1 contains an orthogonalion accelerator arrangement 120 which acts as an ion pusher.Specifically, ions in the ion beam 80, which are travelling along afirst axis upon entering the acceleration and detection chamber 110, arepushed in a generally orthogonal direction by the orthogonal ionacceleration arrangement 120. The result of this arrangement is thatbunches of ions are repeatedly extracted from the ion beam 80 and sentthrough the acceleration and detection chamber 110 towards a detectorarrangement. As will be apparent to the skilled reader, the ion bunchestravel through the acceleration and detection chamber at a velocitywhich is related to the mass-to-charge ratio of the ions. Assuming thata constant electric field is generated by the orthogonal ionacceleration arrangement 120, and that the energy this imparts isconverted to kinetic energy, it may be shown that the ion velocity, v,is inversely proportional to the square root of the mass-to-chargeratio.

Again as will be familiar to those skilled in the art, a reflector array130 may be employed within the acceleration and detection chamber 110 toeffectively double the distance travelled by the ion bunches, and thusto allow better spatial separation of the ions of differing mass-tocharge ratios within separate bunches.

The ions arrive at a detector arrangement 140 where they are detected ina manner to be described in greater detail below. The time of flight ofthe ions is in particular determined, and from this a mass spectrum canbe built up.

Referring now also to FIG. 2, the details of the detector arrangement140 are shown. The detector arrangement 140 comprises a grid or mesh 150formed, for example, from stainless steel, nickel or berillium bronzewith apertures created by electrochemical etching. Ions arrive at thegrid or mesh 150 through a common entrance window to the detectorarrangement 140 and some of the ions strike the mesh itself. Those ionswhich do not strike the mesh pass through it. In this manner, the gridor mesh 150 acts as an ion beam splitter.

Those ions from the incident ion beam which strike the grid or mesh 150generate secondary electrons 160 which are registered by a firstdetector 170. In the arrangement of FIGS. 1 and 2, this first detectorcomprises a micro-channel plate which is a composite electronmultiplier. The secondary electrons 160 which strike the first detector170 are accumulated and then sent to a second data acquisition system180. This data acquisition system may be a TDC, an ADC or a combinationof the two, as is disclosed in the above-referenced WO-A-98/40907, whosecontents are incorporated herein by reference in their entirety.

Those ions which do not strike the grid or mesh 150 pass through it andare then incident upon a second detector 190 which, in the embodimentshown in FIGS. 1 and 2, is again a micro-channel plate. The resultantsecondary electrons are registered by a first data acquisition system200 which may likewise be a TDC, an ADC, or a combination of the two.

The data obtained by the two data acquisition systems 180, 200 may becombined to generate a mass spectrum. The problems of saturation with asingle detector are reduced by the arrangement shown in FIGS. 1 and 2,particularly where the grid or mesh 150 has a substantial number ofapertures distributed across it. Then, the ions impinging upon the gridor mesh 150 are from or across the width of the ion beam, such that eachdetector 170, 190 samples ions distributed across the beam.

It is preferable that a significantly larger proportion of ions passthrough the grid or mesh 150 than strike it. For example, it ispreferable that 90% or more of the ions in the ion beam pass through themesh or grid 150. This is so that one of the two channels (in theembodiment where there are only two channels) keeps counting (when a TDCis used) even when the other channel is already saturated. In thisexample, the second DAS 180 will saturate more quickly than the firstDAS 200, since the bulk of the particles pass through the mesh or grid150 to strike the first detector 190.

The fields necessary to extract the electrons towards the firstmultiplier may lead to TOF aberrations. These may be eliminated by theuse of a compensation electrode 210 due to the symmetry of the geometryin the voltages. Ions passing closer to the compensation electrode 210receive the same TOF aberration as ions passing at the same distancefrom the entrance of the first multiplier. As a result, the TOFaberrations are almost constant across the whole width of the entrancewindow into the multiple detector.

FIG. 3 shows a second embodiment of a dual detector for use in a TOFMS.Features common to FIGS. 2 and 3 are labelled with like referencenumerals.

Instead of separate micro-channel plates arranged orthogonally, as inFIG. 2, the arrangement of FIG. 3 employs distinctly separate and remoteareas of a common micro-channel plate assembly 220. As with thearrangement of FIG. 2, ions enter the detector arrangement through acommon entrance window and a percentage strike the grid or mesh 150. Inthe embodiment of FIG. 3, however, those which strike the mesh generatesecondary electrons 230 which impinge upon a further electron multiplier240. The secondary electrons incident upon the further electronmultiplier 240 generate tertiary electrons 250 which are directedtowards the right-hand side of the common micro-channel plate assembly220 as seen in FIG. 3. The right-hand part of the common micro-channelplate assembly 220 accordingly forms a part of a first detector 170′which is spatially divided from a second detector 190′ as may be seen.Ultimately, the tertiary electrons 250 entering the right-hand side ofthe common micro-channel plate assembly 220 are registered by a firstdata acquisition system which, as with FIG. 2, may be a TDC, an ADC or acombination of the two.

Those incident ions which pass through the grid or mesh 150 are incidenton the left-hand side of the common micro-channel plate assembly 220which forms a part of the second detector 190′. In this case, the ionspassing through the grid or mesh are ultimately registered by a seconddata acquisition system, which may be a TDC, an ADC or a combination ofthe two.

The arrangement of FIGS. 2 and 3 thus separates the incoming ion beam ata much earlier stage than in prior art arrangements; the ion beam isseparated as ions rather than as resulting bunches of electrons.

FIG. 4 shows yet another dual detector arrangement embodying the presentinvention. Here, instead of micro-channel plates, discrete dynodes areinstead employed.

As previously, ions enter the dual detector arrangement via a commonentrance window. The ions approach a first conversion dynode 260 throughwhich a plurality of apertures 270 are formed (see also FIG. 4). In theembodiment of FIG. 4, the first conversion dynode 260 differs from thegrid or mesh 150 of FIGS. 1 to 3 in that the apertures 270 form only asmall fraction of the surface area of the first conversion dynode. Thus,the majority of ions incident upon the first conversion dynode 260 areconverted into secondary electrons 280 which are in turn incident uponan array of electron multipliers 290 which are preferably arranged in aChevron format. The electrons generated by the last of the electronmultipliers 290′ are registered by a first data acquisition system whichmay as previously be a TDC, an ADC or a combination of the two.

That small fraction of ions 300 which pass through the apertures 270 inthe first conversion dynode 260 strike a second conversion dynode 310.As with the grid or mesh 150, the first and second dynodes 260, 310 areformed from stainless steel, nickel, berillium bronze or other suitablematerials. Secondary electrons 320 generated by the second conversiondynode 310 are incident upon a first in a further array of electronmultipliers 330 which are distinct from the array of electronmultipliers 290 that intercept secondary electrons generated by thefirst conversion dynode 260. The electron multipliers 330 are likewisearranged in a Chevron format and the electrons resulting from the lastof the electron multipliers 330′ are registered by a second dataacquisition system which may include a TDC, an ADC or a combination ofthe two. The first conversion dynode 160 allows passage of less than 10%of incident ions and is thus different to the mesh or grid 150 of FIGS.1 to 3 which allows over 90% of ions to pass. The advantage of theconversion dynode over the mesh is that it is easier to manufacture, andthat the secondary electrons 280 (which in the arrangement of FIG. 4represent the bulk of the incident ions) are easier to focus in TOF asthey pass towards the electron multipliers 290 than ions are (becauseelectrons are relatively much lighter).

Preferably, the first and second conversion dynodes 260, 310 are bothperpendicular to the direction of time of flight dispersion. Theincident ions are focussed upon the first conversion dynode 260 and soany that pass through the apertures 270 are subject to an energy spreadε which limits the partial mass resolution R in accordance with theformula$R = {\left( \frac{L}{d} \right) \times \left( \frac{1}{ɛ} \right)}$where L is the total effective path length (here, 1.3 meters) and d isthe gap between the first and second conversation dynodes 260, 310. Foran energy spread of 3% (FWHM) and a required resolution R greater than15,000, d must be less than 2.7 mm. To address this, the arrangement ofFIG. 4 employs a two-stage acceleration as is proposed, for example, byKulikov et al in Trudy FIAN, vol. 155, (1985) pages 146 to 158. Here, anintermediate grid 305 is employed between the first and secondconversion dynodes 260, 310. If an electric field E1 is generatedbetween the first conversion dynode 260 and the intermediate grid 305(to form a first acceleration stage in a gap of length D1), and a secondelectric field E2 is generated in the gap D2 between the intermediategrid 305 and the second conversion dynode 310 (forming a secondacceleration stage), then for (D1)=0.2(D2), TOF focussing is achievedwhen (E2)=0.4(E1). Applying a two-stage acceleration arrangementcircumvents the restrictions imposed on d(=D1+D2) by the formula givenabove and the gap d may be 5 to 10 mm, for example.

The alternative to this arrangement is to reduce the distance d, in thiscase to less than 2.7 mm—in practice a gap of 2.2 mm is preferred. Asuitable arrangement is shown in FIG. 5. Here, the electron multipliers290, 330 are shown simply as blocks for the sake of clarity. However,the first electron multiplier 330 a of the second set of multipliers 330is shown. This electron multiplier 330 a is mounted between the firstand second conversion dynodes 260, 310 because of the limited spaceavailable due to the constraints on the overall gap d. Ions pass throughthe apertures 270 in the first conversion dynode 260 and then throughfurther slots 315 in the first electron multiplier 330 a which arealigned with the apertures 270 in the first conversion dynode 260. Theions then strike the second conversion dynode 310 and secondaryelectrons generated thereby move back towards the first electronmultiplier 330 a. These secondary electrons strike the material of thefirst electron multiplier 330 a between its slots 315 and this in turngenerates tertiary electrons. These are directed back towards the secondconversion dynode which has further slots 325 that do not align with theslots 315 in the first electron multiplier 330 a. The tertiary electronsthus pass through the second conversion dynode and into the electronmultiplier array 330.

Still a further embodiment of a multiple detector is shown in FIG. 6. Aswith the other embodiments, ions 340 enter the detector arrangementthrough a common entrance window from the TOFMS. The bulk of theincident ions 340 strike a first conversion dynode 260′, similar to thefirst conversion dynode in the arrangement of FIGS. 4 and 5. Secondaryelectrons 250 are generated by the first conversion dynode 260′ andthese are accelerated by an accelerating grid 360 away from the firstconversion dynode 260′. The accelerating grid 360 is supplied with apositive potential.

A liner 370 reflects the secondary electron 350 back towards a firstmicro-channel plate 380 which in turn generates tertiary electrons 390.These strike a first scintillator 400 which, as will be familiar tothose skilled in the art, generates photons 410 in response to incidentcharged particles. The photons 410 are captured by a firstphoto-multiplier 420. The ultimate signal is registered by a first dataacquisition system which, as with each of the other embodiments, may bea TDC, an ADC or a combination of the two.

The scintillator may, for example, be formed of barium fluoride or aplastic material such as polyvinyltoluene, with a metallized coatingthat is less than 50 nm thick. With a barium fluoride scintillator, aphotomultiplier having a caesium-tellurium (Cs—Te) photocathode may beemployed, whereas with a plastic scintillator, a photomultiplier with abialkali photocathode is appropriate. If electrons from the back of themicrochannel plate 380 are focussed by electric fields onto the firstscintillator 400, smaller and cheaper scintillators and photomultiplierscan then be used.

It will be noted in FIG. 6 that the first micro-channel plate 380 iscanted at an angle of approximately 60° to the direction of TOFseparation, that is, at approximately 30° to the first conversion dynode260′. This arrangement minimises the time of flight separation, althoughother angles such as 45° may be appropriate.

Those ions 340 which pass through the apertures 270′ in the firstconversion dynode 260′ strike a second micro-channel plate 430.Electrons generated by the micro-channel plate 430 cause a secondscintillator 440 to generate photons 450 which are detected by a secondphoto-multiplier 460. A second data acquisition system, once againcomprising a TDC, an ADC or a combination of the two, registers thephotons arriving at the second photo-multiplier 460. The secondscintillator, photomultiplier and microchannel plate may be formed ofsimilar materials to the first ones.

There are a number of ways of focussing photons from the first andsecond scintillators 400, 440 onto the first and second photomultipliers420, 460 respectively. If the photomultiplier is large enough, nofocussing is necessary. For smaller photomultipliers, a conical lightguide may be used with a polished (e.g. aluminium) inside surface,either in vacuo or at atmosphere (with a fused silica window acting as avacuum seal). Alternatively, a short-focus lens can be employed, whichmay act as a vacuum seal if the photomultiplier is kept at atmosphere.

The advantage of the arrangement of FIG. 6 over other embodimentsdescribed herein is that there is complete galvanic isolation from thenoise of power supplies, switching voltages and so forth. The collectorsof the photomultipliers 420, 460 can also be kept at virtual groundwhich simplifies the preamplifier to which it is connected and alsoreduces its noise. Instead of the chevron arrangement preferred forother embodiments, the microchannel plates 380, 430 in FIG. 6 can besingle stage. The photomultipliers 420, 460 are very sensitive (almostsingle photon) and a single stage plate provides adequate gain.

Although not shown in FIG. 6, it is desirable that the ion entrancewindow to the arrangement of this embodiment has a compensationelectrode similar to the compensation electrode 210 of FIGS. 1 to 3, andfor the same purpose (to minimize ion TOF spread).

Although each of the detectors shown in FIGS. 1 to 5 is a dual detector,it is to be appreciated that three or more detectors can be employedinstead. Likewise, it will be understood that an orthogonal TOFMS isshown in FIG. 1 simply for the purposes of illustration. LongitudinalTOFMS is equally suited to the multiple detector arrangement describedherein. Indeed, the arrangement is also applicable to other forms ofmass spectrometry such as quadrupole mass spectrometry, where oneemploys two counters rather than a counter and an ADC.

1. An ion detection arrangement for a time-of-flight mass spectrometercomprising: an ion beam splitter arranged to block the onward passage ofa first part of an incident bunch of ions which has passed through thetime-of-flight mass spectrometer, but to allow passage of a second partof that incident bunch of ions; a first detector means arranged todetect ions whose passage has been blocked by the ion beam splitter; anda second detector means arranged to detect those ions which pass throughthe said ion beam splitter.
 2. The ion detection arrangement of claim 1,in which the ion beam splitter is arranged to generate secondaryelectrons when ions in the said first part of the ion bunch strike it,whereby the ion beam splitter forms a part of the first detector means.3. The ion detection arrangement of claim 1, in which the first detectormeans further comprises one or more electron multipliers.
 4. The iondetection arrangement of claim 1, in which the second detector meansfurther comprises one or more electron multipliers.
 5. The ion detectionarrangement of claim 3, in which at least one of the electronmultipliers is a micro-channel plate electron multiplier.
 6. The iondetection arrangement of claim 3, in which at least one of the electronmultipliers is a discrete dynode electron multiplier.
 7. The iondetection arrangement of claim 3, in which at least one of the electronmultipliers includes a scintillator and a photo-multiplier.
 8. The iondetector of claim 1, in which the first and second detectors eachcontain a single electron multiplier, the plane of the said firstelectron multiplier being orthogonal to the plane of the said secondelectron multiplier.
 9. The ion detection arrangement of claim 1,further comprising a micro-channel plate assembly which forms a part ofboth the first and second detector means, wherein: a first part of themicro-channel plate assembly is arranged to collect ions that pass, inuse, through the ion beam splitter; and wherein: a second part of themicro-channel plate is arranged to collect secondary electrons resultingfrom those ions that are incident upon the ion beam splitter.
 10. Theion detector arrangement of claim 1, further comprising a microchannelplate assembly which forms a part of both the first and the seconddetector means; wherein: a first part of the microchannel plate assemblyis arranged to collect secondary electrons produced from ions that passthrough the said ion beam splitter, and wherein: a second part of themicrochannel plate is arranged to collect secondary electrons resultingfrom those ions that are incident upon the ion beam splitter.
 11. Theion detection arrangement of claim 10, wherein the second part of themicrochannel plate is arranged to collect secondary electrons resultingdirectly from those ions that are incident upon the ion beam splitter.12. The ion detection arrangement of claim 10, wherein the second partof the microchannel plate is arranged to collect secondary electronsresulting indirectly from those ions that are incident upon the ion beamsplitter.
 13. The ion detection arrangement of claim 1, in which each ofthe first and second detector means comprises a plurality of electronmultipliers each formed from a discrete dynode, and wherein at leastsome of the discrete dynodes in the first and second detector means arearranged as a chevron.
 14. The ion detection arrangement of claim 1, inwhich the ion beam splitter is arranged as a flat plate having aplurality of apertures.
 15. The ion detection arrangement of claim 14,in which the plane of the flat plate is substantially orthogonal to thedirection of TOF dispersion of the ion bunches arriving at the said ionbeam splitter.
 16. The ion detection arrangement of claim 14, in whichthe ion beam splitter is so arranged that the probability ofinterception of incident ions thereby is at least one order of magnitudedifferent to the probability of passage of ions therethrough.
 17. Theion detection arrangement of claim 14, in which the ion beam splitter isa transparent mesh arrangement to generate secondary electrons when ionsare incident thereon, the majority of incident ions passing in usethrough the holes in the mesh.
 18. The ion detection arrangement ofclaim 14, in which the ion beam splitter is a conversion dynode formedwith a series of apertures through which a minority of incident ionspass in use, the majority of incident ions being intercepted by theconversion dynode and converted thereby into secondary electrons in use.19. The ion detection arrangement of claim 1, further comprising acompensation electrode orthogonal to and upstream of the ion beamsplitter.
 20. The ion detection arrangement of claim 1, in which thefirst detector means and the second detector means each furthercomprises a data acquisition system.
 21. The ion detection arrangementof claim 20, in which at least one of the data acquisition systemsincludes a time to digital detector.
 22. The ion detector arrangement ofclaim 20, in which at least one of the data acquisition systems includesan analogue to digital converter detector.
 23. The ion detectionarrangement of claim 4, in which at least one of the electronmultipliers is a microchannel plate electron multiplier.
 24. The iondetection arrangement of claim 4, in which at least one of the electronmultipliers is a discrete dynode electrode multiplier.
 25. The iondetection arrangement of claim 4, in which at least one of the electronmultiples includes a scintillator and a photo-multiplier.
 26. A methodof detecting the time of flight of ions in an ion beam of atime-of-flight mass spectrometer, comprising: directing ions to bedetected through the time-of-flight mass spectrometer and toward an ionbeam splitter; blocking passage of a first portion of the ions in theion beam at the ion beam splitter; allowing passage of a second portionof the ions in the ion beam through the ion beam splitter; detectingions whose passage has been blocked by the ion beam splitter with afirst detector means; and detecting ions passing through the ion beamsplitter with a second detector means.
 27. The method of claim 26,further comprising generating secondary electrons as a consequence ofincidence of ions upon the ion beam splitter, and detecting thesecondary electrons with the first detector means.
 28. An ion detectionarrangement for detecting bunches of ions in a time of flight massspectrometer, comprising: an ion beam splitter arranged downstream ofthe time of flight mass spectrometer and in the path of the bunches ofions, the ion beam splitter defining a plurality of aperturesdistributed across the width of the incident ion bunches; a firstdetector arranged to detect ions which have been passed through the timeof flight mass spectrometer and which then strike the ion beam splitter;and a second detector arranged to detect ions which have passed throughthe time of flight mass spectrometer and which have also passed throughthe plurality of apertures defined by the ion beam splitter.
 29. The iondetection arrangement of claim 28, wherein the ion beam splitter is asubstantially transparent mesh, whereby the majority of ions in eachbunch that passes through the time of flight mass spectrometer also passthrough the apertures in the mesh and only a minority of the ions fromthe time of flight mass spectrometer strike the mesh structure.
 30. Theion detection arrangement of claim 28, wherein the mesh is so configuredthat at least 90% of the ions arriving at the mesh from the line offlight mass spectrometer pass through the apertures therein.
 31. The iondetection arrangement of claim 28, wherein the ion beam splitter is aplate defining a plurality of apertures, and wherein the relativedimensions of the plate and the aperture defined therein are such as topermit passage of only a minority of the ions from the time of flightmass spectrometer through the said apertures, to the second detector,the majority of the said ions from the time of flight mass spectrometerstriking the plate.